This invention relates to an optical coupling module with an integral attenuator for coupling a light beam between a source and a receiver. This invention also relates a method for forming the optical coupling module.
Known optical coupling modules for use in optical transmitters, receivers and transceivers typically include a number of elements that need to be aligned and assembled using specially built production equipment. Alignment and assembly of such optical coupling modules is costly. FIG. 1 shows a prior art optical transmitter having such a design. Consequently, an objective in the design of optical coupling modules is therefore to limit the number of components and associated alignment steps. The optical transmitter includes a header received in an optical coupling module. The header supports a laser therein. The header has a window through which laser light is emitted. The optical coupling module includes a lens and a fiber receptacle. The laser, lens and fiber receptacle need to be accurately aligned. U.S. Pat. No. 5,937,114 discloses another optical coupling module that includes optical and mechanical piece parts or elements that have to be accurately fabricated and assembled to prevent tolerance build-up.
The assembly process for these known optical coupling modules might be further complicated by the need to include an attenuator for attenuating a light beam that propagates through each of the optical coupling modules. For example, when an optical coupling module is a part of an optical receiver, there might be a need to attenuate a light beam received from an optical fiber so as to match the power range of the light beam with the dynamic range of an optical detector. Similarly, the introduction of an attenuator in an optical transmitter typically limits the optical power launched into an optical fiber to be within specifications according to a respective fiber communication standard. The attenuator is employed in the optical transmitter to allow a laser in the optical transmitter to be driven with a current that allows optimal dynamic performance of the laser, while limiting the optical power launched into an optical fiber. One typical parameter that is often optimized by increasing the average laser drive current is the laser relaxation oscillation frequency.
In accordance with a known method, the intensity of a light beam that is launched into a fiber may be reduced by shifting an element of the optical coupling module away from its optically aligned position, where the optically aligned position results in maximum coupling efficiency by the elements of the optical coupling module. For instance, the launched power can be tuned by shifting the fiber either closer or further away from the laser. This intentional shift, away from an optimum position, has the disadvantage that it causes the optical coupling module to operate under sub optimal coupling efficiency conditions, as indicated by region X, as shown in FIG. 2. When operating in region X, the optical coupling module is more sensitive to external disturbances, such as temperature changes, as compared to a module that is aligned for maximum coupling efficiency, as indicated by point Y in FIG. 2. Consequently, a small amount of thermal expansion of the elements of the optical coupling module is likely to cause a relatively large change in the optical power emitted by the laser that is launched into an optical fiber, which might exceed specifications set by the communication standard.
Another known method of attenuating a light beam is by deposition of a reflective coating on one or more surfaces of one or more optical elements of an optical coupling module. For example, the window and the lens of the optical transmitter in FIG. 1 may have a reflective coating thereon. Reflective coatings on glass surfaces are common for providing attenuation, however such coatings can be costly. Furthermore, a reflective coating has the disadvantage in that its optical transmittance might depend on the state of polarization of the light beam, which will limit its usability. In addition to being costly and polarization sensitive, another disadvantage of applying a reflective coating on a polymer surface is that at elevated temperatures, cracks might appear in the coating layers due to the difference in thermal expansion of the coating layer and the polymer. These cracks will disturb a light beam impinging on the reflective coating and thereby affect transmission of the light beam therethrough.